Process for making a solid state cell

ABSTRACT

A solid state electrochemical laminar single cell comprising: 
     an alkali metal anode, 
     a solid ionically conducting electrolyte, 
     a composite cathode, and 
     a current collector, 
     wherein said electrolyte is interposed between said alkali metal anode and said cathode and said cathode is interposed between said electrolyte and said current collector, 
     said cell being capable of generating a discharge current per unit area greater than or equal to 50 milliamperes/cm 2  at room temperature; and a process for producing the cell is disclosed.

This is a divisional of application Ser. No. 343,976, filed Apr. 26,1989.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacture of a solid stateelectrochemical cell, and more particularly, a rechargeable solid statelaminar cell including a lithium anode which is capable of generating ahigh current density.

2. Description of the Prior Art

Solid state electrochemical rechargeable chemical cells are the subjectof intense investigation and development. They are described extensivelyin the patent literature. See, for example, U.S. Pat. Nos. 4,303,748 toArmand; 4,589,197 to North; 4,547,440 to Hooper, et al; and 4,228,226 toChristian. These cells are typically constructed of an alkali metal foilanode, typically lithium foil, an ionically conducting polymericelectrolyte, a composite cathode containing a finely divided transitionmetal oxide and an electrically-conductive filler, and a currentcollector. These cells present a viable option to older, moretraditional, secondary cells because of their ability to generate arelatively high current per unit area and high storage capacity.

To date, solid state lithium anode batteries which have been describedin the literature have only been capable of discharging current at arate of between about 0.05 ma/cm² and 10 ma/cm². Efforts are continuingto be made to improve the efficiency of these cells.

According to Ohm's law, the voltage drop of a cell is equal to thecurrent multiplied by the cell's internal resistance (impedance).Accordingly, for a solid state lithium cell having a voltage of 2.7volts, if the overall impedance of the cell is reduced, the amount ofcurrent available from the cell is correspondingly increased, making ita more attractive commercial alternative.

In solid state alkali metal anode laminar cells, internal cell impedanceis the product of a number of factors. An internal impedance isassociated with each element of the cell, namely the anode layer, theelectrolyte layer, the cathode layer, and current collector. Further,and is particularly problematic, high impedance can occur at theinterfaces between these layers and, more specifically, at theanode/electrolyte interface, at the electrolyte/cathode interface and atthe cathode/current collector interface. Accordingly, to produce anefficient solid state electrochemical laminar cell which is capable ofdelivering high current density and storage capacity, the impedance ofeach of these layers and interfaces must be minimized. The presentinvention seeks to provide as small an internal impedance as possible.

In addition to providing a cell having a minimum internal impedance, itis also desirable to produce a cell capable of operating across a widetemperature range. For example, presently available solid state lithiumanode laminar cells operate very inefficiently at or below roomtemperature.

Accordingly, there exists a need in the art for a solid state alkalimetal anode laminar cell which has a low overall impedance, and iscapable of discharging a high amount of current per unit area over abroad temperature range.

SUMMARY OF THE INVENTION

The present invention relates to a solid state electrochemical alkalimetal anode laminar cell capable of discharging current at anunexpectedly high rate per unit weight and volume. The cells of thepresent invention are capable of discharging current at a rateapproximately 3 to 10 times greater than cells previously disclosed inthe literature. The cells of the present invention are furthercharacterized in being able to maintain a high current discharge rate attemperatures below room temperature.

The high energy output of the cells of the present invention is obtainedby minimizing overall cell impedance. This is accomplished by minimizingimpedance in each of the cell layers, as well as minimizing theimpedance across the interfaces between the layers. Maintaining aminimum cell impedance in accordance with the present invention isaccomplished by appropriate selection of materials used for forming thecomponent layers of the cell, selection of optimal layer thicknesses foreach of the component layers, and using manufacturing techniques whichminimize interfacial impedance.

A further consideration is that the cell be operative across a widetemperature range. The inventors have discovered that by minimizing cellimpedance, the resulting cell may be used across a relatively broadtemperature profile, including temperatures below room temperature.

In accordance with the present invention, a solid state electrochemicallaminar cell is provided. The cell includes:

an alkali metal anode;

a solid ionically-conducting electrolyte;

a composite cathode; and

a current collector;

wherein said electrolyte is interposed between said alkali metal anodeand said cathode, and said cathode is interposed between saidelectrolyte and said current collector; said cell being capable ofgenerating a discharge current per unit area greater than or equal to 50ma/cm² at room temperature.

In practice, the preferred anode is a lithium foil or a lithium-coatedmetal foil. The preferred electrolyte is a single-phase solid solutionof an ionizable alkali metal salt, a solvent for the salt, and a polymerwhich has been polymerized by exposure to actinic radiation, heat orwhich has been chemically polymerized. The cathode compositionpreferably includes V₆ O₁₃, electrically-conductive carbon particles andthe above-described ionically-conductive electrolyte. The preferredcurrent collector material is a nickel foil having particles of nickelelectrochemically deposited on the surface of the foil which contactsthe cathode composition Further, by minimizing the thickness of thelayers so as to minimize the thickness-per-surface area ratio for eachlayer, the overall thickness of the cell can range from about 130 toabout 350 microns.

In accordance with another embodiment of the present invention, the cellis produced by a process comprising the steps of:

coating a radiation polymerizable cathode composition onto a currentcollector;

coating an ionically-conductive polymerizable electrolyte compositiononto said cathode composition;

partially or totally curing said cathode composition and saidelectrolyte composition; and

applying an alkali metal anode onto said electrolyte composition to forma cell assembly.

In a particularly preferred embodiment, the surface of the anode layerwhich contacts the electrolyte composition is smoothened to remove orbreakdown a passivation layer prior to coating. Further, the exteriorsurface of cathode composition is preferably rolled after coating ontothe current collector to provide a smooth surface for the electrolytecomposition to be coated thereon. In addition, in an embodiment wherethe electrolyte and cathode are partially cured, the entire assembly issubjected to an overall curing step after application of the alkalimetal anode layer onto the electrolyte layer.

Utilization of the above method accomplishes the goals of minimizing thethickness of each layer, maximizing electrical and ionic contact betweenlayers, and reducing the internal impedance. Accordingly, the resultingcell is capable of generating a high current per unit area.

Accordingly, it is an object of the present invention to produce a solidstate electrochemical laminar cell capable of high energy output as aresult of having a low cell impedance.

A further object of the present invention is to provide a solid stateelectrochemical laminar cell capable of high energy output wherein thecell is rechargeable.

Still another object of the present invention is to provide a solidstate electrochemical laminar cell capable of generating a high currentper unit area at temperatures lower than room temperature.

These, and other objects, will be readily understood by one skilled inthe art, as reference is made to the following drawings and detaileddescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cut-away view of a cell embodying the teachings of theinstant invention.

FIGS. 2(a)-(e) depict a process for producing the cell of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

While describing the preferred embodiment, certain terminology will beutilized for the sake of clarity. It is intended that such terminologyinclude not only the recited embodiment, but all technical equivalentswhich perform substantially the same function, in substantially the sameway to achieve substantially the same result.

A laminar solid state cell produced in accordance with one embodiment ofthe present invention is shown in FIG. 1 and is represented by element10. Cell 10 includes current collector layer 12, cathode compositionlayer 14, electrolyte composition layer 16 and alkali metal anode layer18. Cell 10 also includes protective covering 20 which functions toprevent water and air from contacting the reactive layers of the cell.

Cell 10 is preferably a laminar thin cell type including a lithiumanode. Laminar thin-cell batteries containing lithium anodes are knownin the art, and it will be appreciated that the cell can include variousconstructions such as bi-faced or bipolar cell designs. Examples of cellconstructions include a "jelly roll" or a fan folded laminate stripdesign, both of which are illustrated in U.S. patent application Ser.No. 238,071 filed Aug. 30, 1988, which is hereby incorporated byreference.

Cell 10 is particularly characterized by its ability to discharge alarge amount of current per unit area. Values equal to and exceeding 50ma/cm² at room temperature have been achieved. This can correspond to anenergy density greater than 100 watt hours/liter and a power densitygreater than 100 watts/liter. In accordance with the present invention,energy densities exceeding 300 watt hours/liter and power densitiesexceeding 300 watts/liter are possible.

The high energy/power of the cell is accomplished by maintaining anoverall cell impedance that is low as possible. In practice, the overallimpedance should be less than 100 ohm-cm⁻² and preferably less than 50ohm-cm⁻². To achieve such a low overall impedance, it is important tominimize the impedance of the component layers of the cell and theimpedance of the interfaces between the respective layers. To furtherminimize overall impedance, the cell is produced by a specially designedprocess.

The process for producing cell 10 is depicted in FIG. 2. Because thecell produced in accordance with the process of FIG. 2 utilizes analkali metal anode layer, usually a lithium anode layer, it is necessaryto manufacture the cell in a water (humidity) free environment. Lithiumis extremely reactive with water and if reacted, a passivation layer canform on the surface of the anode layer, reducing the efficiency of thelayer, and increasing cell impedance. Accordingly, it is particularlydesirable to manufacture the cell in an environment having a relativehumidity at room temperature of less than 2% (less than 300 ppm water).An environment containing between 1 ppm and 50 ppm water produces aparticularly efficient cell.

Referring to FIG. 2(a), cathode current collector 12 is provided.Collector 12 may be a sheet of metal foil such as aluminum, nickel orstainless steel, an electrically conductive screen, grid, foamed orexpanded metal, etched foil, electrodeposited film, woven or non-wovenconductive fabric In practice, a thin metal foil having a metal,preferably the same metal, electrodeposited on the surface which willcontact the cathode layer is preferred. A preferred metal is known assurface-treated nickel. As seen in FIG. 2(a), the electrodepositedparticles of nickel are designated as element 13. A microroughened oretched surface enables better adherence of the cathode composition tothe current collector.

In practice, the thickness of current collector 12 ranges from about 5microns to about 25 microns. Preferably, the current collector is asthin as practicable. Alternatively, the current collector can take theform of a thin polymeric film having coated thereon an electricallyconductive metal. The advantage of this current collector is that it isextremely light-weight and can be utilized in extremely thin layers. Anexample of such a material is a polyethylene terephthalate substratehaving electrodeposited thereon a first layer of copper and a secondlayer of nickel. The thickness of such a layer is typically about onemicron but it may be as thin as practicable to minimize overallthickness of the cell.

Referring to FIG. 2(b), a cathode composition 14 is coated onto themicroroughened surface of current collector 12. Cathode composition 14is paste-like in consistency and typically includes an active cathodicmaterial such as a transition metal chalcogenide, an intercalationcompound, or an electrically conductive polymer such as polypyrrole,polyacetylene, polyaniline, etc.; an electrically conductive filler; andan ionically-conductive curable electrolyte (described below).

Intercalation compounds and electrically conductive materials useful inthe present invention are known in the art. Representative examples oftransition metal oxides and sulfides useful as active cathode materialsare V₆ O₁₃, V₂ O₅, Mo0₂, TiS₂, MnO₂, V₂ O₅, MoS₃, Cr₃ O₆, Li_(x) V₃ O₈,FeS, NiS, CoO and CuO. Other examples are described in the literature.The active cathode material preferably has a particle size of less than1 micron but can range up to 20 microns.

A particularly preferred intercalation compound is V₆ O₁₃ having aparticle size less than 5 microns, and particularly less than onemicron. V₆ O₁₃ comprises between 25 and 75% by weight of the entirecathode composition, and preferably between 45 and 65%.

The electrically conductive filler is usually carbon particles. Certainconductive polymers (which are characterized by a conjugated network ofdouble bonds) like polypyrrol and polyactylene may also be used for thispurpose.

It is particularly desirable to maintain good contact between thecathode material and the carbon particles while maximizing the amount ofcathode material available for ionic interaction. If the ratio of acathode material such as V₆ O₁₃ to carbon is greater than 10 to 1 byweight, the cathode composition provides poor electronic conductivitydue to the lack of carbon. Conversely, if the ratio is less than 5 to 1,an excess of carbon is present which does not significantly improveelectrical conductivity but does reduce capacity due to the reducedamount of intercalation compound present.

The optimum ratio of carbon to a transition metal chalcogenide will varywith particle size and processing or grinding techniques used.Preferably the carbon black and metal oxide are ball milled together.The preferred ratio is between 6 to 1 and 7 to 1. Moreover, unless theratio of V₆ O₁₃ to carbon is maintained between 5:1 and 10:1, it is verydifficult to coat the cathode composition onto the current collector. Iftoo much carbon is present, the cathode composition has a noncompactedsand-like consistency.

It has further been discovered that the impedance of the cathodecomposition is reduced when the particle size of the intercalationcompound and the carbon filler is approximately equal, i.e., from lessthan 1 micron to about 5 microns. This can easily be accomplished bymixing and grinding the two components together.

The third component of the cathode composition is an ionicallyconductive electrolyte. This material is typically prepared from acomposition which comprises a liquid, monomeric or prepolymeric curablecompound a solution of an ionizable alkali metal salt whichinterpenetrates the network of the formed polymer.

Ionizable alkali metal and alkaline earth salts useful in theelectrolyte include those salts conventionally used in solid stateelectrochemical cells. Representative examples are Li⁺, Na⁺, K⁺, Mg²⁺,Ca²⁺, and ammonium salts of less mobile anions of weak bases having alarge anionic radius. Examples may be selected from the group consistingof I-, Br-, SCN-, ClO₄ -, BF₄ -, PF₆ -, AsF₆ -, CF₃ COO-, CF₃ SO₃ -, CF₃CO₃ -, B₁₂ H₁₂ ²⁻, B₁₀ Cl₁₀ ²⁻, and BO₄ -, where O is C₆ H₅, an alkylchain or an aryl chain, and wherein said salt cation and said salt anionare maintained in stoichiometric amounts. Specific examples are LiAsF₆,LiClO₄, NaClO₄, LiF₃ SO₃, and LiBF₄. LiAF₆ is a particularly preferredsalt as is it capable of being used in relatively high amounts.

The solvent for the salt can be any low volatile aprotic polar solvent.Preferably, these materials are characterized by a boiling point greaterthan about 80° C. If the polymeric network is formed by radiationpolymerization, the solvent should be inert to the radiation and if thenetwork is formed by thermal polymerization, the solvent should bethermally inert. In particular, the solvent should not scavenge freeradicals. Low volatility simplifies manufacture and improves shelf life.Representative examples are propylene carbonate, -butyrolactone,1,3-dioxolane, and 2-methyltetrahydrofuran. Less polar solvents havingheteroatoms capable of bonding alkali metal cations are also useful.Polyethylene glycol dimethyl ether (PEGDME) is one such example. Glymessuch as tetraglyme, hexaglyme, and heptaglyme are also desirablesolvents Propylene carbonate is a preferred solvent.

Polymerizable compounds useful in the electrolyte composition may yieldeither a conductive or non-conductive polymer. Compounds which yield aconductive polymer contain a heteroatom capable of formingdonor-acceptor bonds with the alkali metal cation. Useful polymerizablecompounds are described next.

Polyethylenically unsaturated monomeric or prepolymonomeric materialsuseful in the present invention are preferably compounds having at leastone, and more preferably a plurality, of heteroatoms (particularlyoxygen and/or nitrogen atoms) capable of forming donor acceptor bondswith an alkali metal cation and are terminated by polymerizablemoieties. These compounds yield a conductive supportive matrix. Morespecifically they are preferably low molecular weight oligomers of theformulae (I)-(III) below ##STR1## where n is about 3 to 50 and R ishydrogen or a C1-C3 alkyl group, which are terminated by ethylenicallyunsaturated moieties or glycidyl moieties represented by A.

A particularly useful group of polymerizable compounds is obtained byreacting a polyethylene glycol with acrylic or methacrylic acid. Alsouseful in the present invention are curable materials such as acrylatedepoxies, e.g., Bisphenol A epoxy diacrylate, polyester acrylates,copolymers of glycidyl ethers and acrylates or a vinyl compound such asN-vinylpyrrolidone. The latter provide a non-conductive matrix.

The curable electrolyte mixture of this invention contains at least 45%by weight of the radiation inert liquid and about 20 to 55% by weightand preferably 25 to 40% by weight of the polymerizable compound. Theexact amount of the polymerizable compound and the solvent should beadjusted to provide the optimum combination of strength and conductivityfor the particular application. As a general rule, if the mixturecontains less than about 20% of the polymerizable compound, theelectrolyte will be too weak to maintain electrode separation. If themixture contains greater than about 55% polymerizable material, theelectrolyte exhibits poor conductivity. In those cases in which theelectrolyte composition itself or an electrode composition containingthe electrolyte is coated on a supporting member, such as a currentcollector or an electrode half element, the electrolyte often is notrequired to have the structural integrity of a free standing film. Inthose applications it is permissible and advantageous to use a higherquantity of the solvent because greater conductivity can be achieved,for example it is advantageous to use about 70 to 80% of the solvent.

Preferably, the aforementioned polymerizable polyethylenicallyunsaturated compounds have a molecular weight of about 200 to 2,000 andmore preferably 200 to 800. Still more preferably they are liquids attemperatures less than 30° C. Examples of curable materials includepolyethylene glycol-300 diacrylate (average PEO molecular weight about300), polyethylene glycol-480 diacrylate (average PEO molecular weightabout 480) and the corresponding methacrylates.

It may be desirable to include a curable comonomer in the composition toreduce the glass transition temperature and improve the conductivity ofthe polymer. Any suitable monoacrylate such as tetrahydrofurfurylacrylate, tetrahydrofurfuryl methacrylate, methoxypolyethylene glycolmonomethacrylate, 2-ethoxyethyl acrylate, 2-methoxyethyl acrylate orcyclohexyl methacrylate may be used for this purpose. Triacrylates suchas trimethylolpropopane triacrylate (TMPTA), trimethylolpropaneethoxylated triacrylates (TMPEOTA) or trimethylolpropanepropoxytriacrylate may be used to introduce crosslinking of the polymer.Monoacrylates may be used in an amount of about 5 to 50% by weight basedon the total amount of polymerizable material. The triacrylates are usedin amounts of about 2 to 30% by weight on the same basis.

The polymerizable electrolyte is polymerized by radiation, heat, orchemical methods. In a particularly-preferred embodiment, theelectrolyte is cured by exposure to an electron beam. The electron beamis capable of generating free radicals and initiating polymerizationwithout any photoinitiator. Other forms of radiation, such asultraviolet radiation, may require a photoinitiator. Similarly, if athermal curing method is utilized, a thermal initiator should beselected. Examples of thermally-curable electrolytes are set forth inU.S. Pat. No. 4,792,504 to Schwab et al., which is hereby incorporatedby reference.

Referring to FIG. 2(b), cathode composition 14 is coated as a thin filmonto current collector 12. Coating may be accomplished usingconventional coating techniques such as doctor blade or an extrusionmethod. In practice, the optimum thickness ranges between about 25 andabout 250 microns. In order to obtain a one-hour discharge with 75-100%utilization of the cathode composition, the layer thickness rangesbetween 50 and 100 microns in thickness. Where a faster discharge rateis desired, a lower thickness may be selected. Conversely, if a slowerdischarge rate is desired, a thicker layer may be provided. In practice,the cathode composition layer has an impedance less than 50 ohms/cm².

Referring to FIG. 2(c), after cathode composition 14 has been coatedonto current collector 12, surface 15 is rolled in the direction ofarrow A by utilizing a non-stick pressure roller P, such as a PTFEroller. Alternatively, if the cathode composition sticks to the surfaceof roller P, a non-stick release liner, not pictured, may be placed ontosurface 15, roller P can traverse the length of the release liner, andthe release liner can be removed. In still another embodiment, therolling step may involve the utilization of a continuously-operatingsmooth surface belt which upon contact with the cathode compositionsmoothens its external surface. If this technique is performed, the beltpreferably has a mirrored surface. Rolling surface 15 provides benefitsin that it improves adherence between cathode composition 14 and currentcollector 12 and produces a smoother surface which enables a very thinelectrolyte layer to be coated thereon. As a result of rolling surface15, the open circuit voltage discharge associated with cathodecomposition 14 is significantly reduced as compared to a cell whosecathode does not have a rolled surface. Accordingly, the overall cellefficiency is improved.

The combination of utilizing microroughened surface of current collector12 along with the pressure rolling step following the coating of cathodecomposition 14 onto current collector 12 enables a tightly adherentcontact to occur between the respective materials. This, in turn,reduces impedance at the collector/cathode composition interface. Theimpedance at the interface is typically less than 10 ohms/cm² and, inthe preferred embodiment, less than 5 ohms/cm².

Referring now to FIG. 2(d), electrolyte layer 16 is coated as a thinfilm onto surface 15. Electrolyte layer 16 must be an ionicallyconductive composition. In the preferred embodiment, electrolyte layer16 comprises a solution of an alkali metal salt in a radiation inertliquid, and a radiation polymerizable material. Alternatively, athermally-curable or chemically-curable electrolyte may be selected. Anelectrolyte identical to that disclosed above with respect to thecathode composition is particularly preferred (e.g., a solution ofLiAsF₆ in propylene carbonate in a radiation polymerizable monomer suchas polyethylene oxide diacrylate).

An important feature of the cured electrolyte is that it remains asingle phase electrolyte across a broad temperature band. When oneusually compounds a carrier such as propylene carbonate and a polymersuch as polyethylene oxide, the resulting material is a two-phasecomposition. Phase separation is particularly pronounced when thematerial is cooled below room temperature. By comparison, the curablecomposition used herein produces a single phase electrolyte. In thepresent invention, the monomer, salt and inert liquid are mixed togetherand coated. The mixture, when cured, forms a highly crosslinkedpolymeric network containing the alkali metal salt solution. Thepolymer, salt and, solvent form a homogenous, single phase which ismaintained upon curing, and does not readily separate upon cooling totemperatures below room temperature. The highly stable, single phasecharacteristic of the electrolyte contributes to low impedance.

A further feature which helps reduce the impedance of the electrolytelayer is the selection of the type and amount of salt maintained insolution. In general the salt is used in an amount approaching itssolubility limit in the electrolyte. The compositions disclosed here arecapable of dissolving high amounts of salt. LiAsF₆ is a particularlygood ionic conductor having a low impedance. The amount of salt andparticularly LiAsF₆ maintained in the electrolyte typically ranges fromabout 6% to about 24%, with amounts between 15 and 20% beingparticularly preferred. It is hypothesized that high amounts of salt maybe maintained in the electrolyte as a result of the use of the curablemonomer matrix as opposed to compounding the electrolyte from apreformed polymer.

The overall impedance of the electrolyte is about or less than 300ohm-centimeters. The electrolyte can be extruded and coated in a verythin layer, typically ranging from about 5 microns to about 25 microns.The ability to coat in a thin layer is in large part due to thecontinuous surface of cathode composition 15. When electrolyte 16 iscoated onto surface 15, it is coated in an uncured liquid state. As isreadily understood, electrolyte layer 16 must completely coat cathodecomposition layer 14 to prevent the intercalation compound andelectrically conductive filler from protruding through electrolyte layer16 and increasing the open circuit discharge voltage. As seen in FIG.2(b), when cathode composition 14 is initially coated on currentcollector 12, the upper surface of cathode composition 14 is slightlyirregular, having areas of higher protrusion. If not rolled, a thickerlayer of electrolyte 16 would have to be coated to completely cover thesurface irregularities and thereby prevent the intercalation compoundand electrically conductive filler from contacting the anode layer toprevent spontaneous discharge. As increased thickness increasesimpedance of the electrolyte layer, the rolling of surface 15 provides acontinuous coating surface for electrolyte layer 16. The thickness ofelectrolyte layer 16 need only be thick enough to completely coatsurface 15.

The impedance between the cathode composition and electrolyte interfaceis extremely small, being less than 25 ohm-cm². It is hypothesized thatthe low interface impedance is obtained by the use of compositionshaving identical electrolytes, by maintaining a smooth coating surfacefor the electrolyte, by coating both cathode composition and electrolytein a liquid state and by allowing a minimal amount of interfaceintermixing to occur.

Once electrolyte layer 16 has been coated onto surface 15, the assemblyis partially or totally cured, for example, by exposure to actinicradiation. In practice, an electron beam operating at a power of 3 to 9Mrad is particularly preferred. Alternatively an ultraviolet source maybe selected. If an ultraviolet source is selected, the monomerpreferably includes an ultraviolet initiator of the type commonly knownin the art such as thioxanthone initiators. Alternatively, a thermalsource may be selected if the monomer includes a thermal initiator.Curing the cathode composition and the electrolyte polymerizes andcrosslinks and thereby solidifies the monomeric material by conversionto a polymeric form.

Of the above factors leading to a reduced interface impedance, thefactor that both the cathode composition and the electrolyte layers arecoated as liquids is of particular significance. This method is referredto as "wet on wet" coating in that both the cathode composition andelectrolyte layers are coated as liquids and are maintained as liquidsuntil the curing step. By comparison, batteries prepared using a "wet ondry" coating procedure generally result in higher interface impedance.

A partial curing step (as opposed to full curing) may be particularlydesirable as this enables the electrolyte layer 16 to remain somewhattacky. This enables better adherence between the electrolyte and theanode layer, when coated.

After partial or total curing of cathode composition 14 and electrolyte16, alkali metal anode layer 18 is applied to electrolyte layer 16.Although not pictured, a thin polymeric material such as a porouspolypropylene sheet may be interposed between the anode and theelectrolyte to ensure that the anode does not contact the cathode layer,particularly at the outer edges of the respective layers. Use of thepolymeric sheet is optional. Prior to coating it is particularlydesirable to pretreat surface 19 of anode layer 18 to remove any oxidepassivation layer or dendrites which may have been produced thereon. Theeasiest way to pre-treat surface 19 is to apply pressure from anon-stick pressure roller across the length of surface 19. In the casewhere anode layer 18 is lithium metal or a metal having a layer oflithium deposited thereon, removal of the passivation layer is visiblydiscerned by the transformation of the color of surface 19 from dullgray to shiny silver.

Alkali metal anode 18 may take the form of a lithium foil, alithium-coated foil such as nickel or copper foil having a layer oflithium deposited on its surface or a lithium alloy. Examples of lithiumalloys include Li-Al, Li-Si, Li-Sn, Li-Cd, Li-Zn, Li-Pb and Li-N. Forsome applications, a lithium alloy may be preferred because the mobilityof lithium in the alloy may be improved, thereby preventing surfacedendrite formation. However, in the most typical embodiment, a lithiumfoil or lithium-coated metal foil is used. Lithium is a preferred anodematerial because it is very electropositive and light in weight.However, other electropositive alkali metal materials, such as sodium,may be practiced within the scope of the invention.

It is desirable to minimize the thickness of the anode layer to enablethe production of as thin a cell as possible and to minimize theelectrolyte thickness to surface area ratio. However, because metalfoils have low impedance by comparison to the balance of the cellcomponents, this is advisable but not essential. The layer theoreticallyshould be no thicker than necessary to supply lithium ions to theelectrolyte and to provide a surface for replating the lithium metal. Inpractice, commercially available foils are used which range from about50 to 125 microns.

As discussed above, it is preferred that surface 19 be continuous toenable it to uniformly transfer electricity during both charging anddischarging cycles. However, if it is desired to produce anonrechargeable primary discharge cell, surface 19 may be microroughenedto provide a greater surface area for contacting electrolyte 16.

The alkali metal anode is extremely reactive with water. Another factorwhich has been found to contribute to overall low impedance is tomanufacture the battery in a dry room or an environment containing lessthan 450 ppm water and preferably less than 1 or 2 ppm water.

After anode layer 18 is coated onto electrolyte 16, the entire assemblyis passed through pressure rollers P in the direction of arrows B asshown in FIG. 2(e). The pressure rolling step aids in the adhesion ofthe layers to each other,. thereby reducing interfacial impedancebetween component layers.

If the cathode composition 16 and electrolyte 14 have not beencompletely cured, the entire assembly is again cured in the preferredembodiment by exposure to actinic radiation, preferably electron beamradiation. This step functions to solidify the cathode composition andelectrolyte layers, thereby producing a solid state cell.

Because of the rolling of surface 19, the overall rolling of thecomponent layers, and the optional overall curing step, a tightlyadherent contact between electrolyte layer 16 and anode layer 18 occurs.This reduces impedance at the anode/electrolyte interface. In accordancewith the preferred embodiment, the impedance at the anode/electrolyteinterface is less than 100 ohms/cm².

Once the current collector, cathode composition, electrolyte compositionand anode composition have been assembled, electrodes are attached tothe anode and current collector layers by means known in the art. Theassembly is then inserted into an air and water impermeable protectivematerial and the edges of the protective material are sealed, preferablyby heat sealing around edges of the cell components. Sealing preferablyoccurs under vacuum conditions to enable the protective material to forma tightly adherent seal around the component layers and electrodes suchthat the only external access to the component layers is via theelectrodes.

Examples of heat sealable gas and water impermeable protective materialsinclude a multi-layered material having an interior heat sealable layercomprising ethylene acrylic acid, an intermediate barrier layercomprising aluminum foil, and an exterior layer of polyethyleneterephthalate. Other heat sealable protective materials known in the artcan be used in accordance with the present invention. The protectivematerials should be as thin as possible to minimize the overallthickness of the cell. Commercially available heat sealable materials ofthe types described above can have an overall thickness of less than 200microns.

Once the components have been inserted and sealed in the protectivematerial, the cell is available for use by simply connecting theelectrodes to the device to be powered. When utilizing a single lithiumanode cell, the cell generates a voltage of approximately 2.7 volts anda current flow exceeding 50 milliamps/cm². A further feature of the cellof the present invention is that it is capable of generating a largecurrent flow at below room temperatures.

The invention is illustrated in more detail by the followingnon-limiting example

EXAMPLE

In an inert, nitrogen environment, a cathode composition is prepared bygrinding 300 parts of V₆ O₁₃ for 24 to 48 hours in the present of anequal amount of tetrahydrofuran in a one liter ball mill using 1.25 cmdiameter ceramic balls. After grinding, the average particle size of V₆O₁₃ is about 1.5 microns. The slurry is transferred to an airtight mixerand 46.2 parts of predried carbon are added to the mixer to produce aslurry having a weight ratio of V₆ O₁₃ to C of about 6.5 to 1. Themixture is stirred at low speed (20 rpm) under vacuum and heat, untilthe tetrahydrofuran is evaporated. The overall water content of themixture is less than 100 ppm. 3 parts of polyethylene oxide (PEO) havinga molecular weight greater than 100,000 is added to the V₆ O₁₃ /C powdermixture. The mixture is stirred for about 10 to 20 minutes to adequatelydisperse the polyethylene oxide. Propylene carbonate (PC), polyethyleneglycol dimethyl ether (PEGDME), and trimethylolpropane ethoxylatedtriacrylate (TMPEOTA) are added to the mixture to produce a mixturehaving the following components:

    ______________________________________                                        component     percent (weight)                                                ______________________________________                                        V.sub.6 O.sub.13                                                                            45                                                              C             7                                                               PC            37                                                              PEO           1                                                               PEGDA         8.5                                                             TMPEOTA       1.5                                                             ______________________________________                                    

The mixture temperature is increased to 65° C. and the mixture isstirred at low speed for 20 minutes. The speed of the mixer is increasedto 75 rpm and the mixture is stirred for 2 to 3 additional hours.

The mixture is then coated onto a 5 cm×20 cm×25 micron high surfacetreated nickel foil available from Fukuda Metal Foil & Powder Co. Ltd.of Kyoto, Japan by utilizing a doctor blade technique at 50°-60° C. in acompletely inert (nitrogen) environment containing less than 25 ppmwater. The thickness of the cathode layer is 75 microns and the cathodelayer is then covered with a stainless steel foil. A 2 inch diameter, 10inch long stainless steel roller is placed on top of the foil and theroller is rolled along the length of the foil at a pressure of 5-10kg/cm² to improve adherence of the cathode layer to the currentcollector. The assembly is then irradiated with a 3 Mrad dose ofradiation by utilizing an electron beam source to cure the cathodelayer. The foil is then peeled off of the cathode layer. The impedanceof the cathode is less than 50 ohm/cm².

An electrolyte is prepared by mixing together the following componentsin the following weight fractions:

    ______________________________________                                        component     percent (weight)                                                ______________________________________                                        PC            68                                                              LiAsF.sub.6   18                                                              PEO           2.5                                                             PEGDA         9.2                                                             TMPEOTA       2.3                                                             ______________________________________                                    

The overall water concentration of the electrolyte is less than 50 ppm.The electrolyte is coated onto the cathode layer by using a doctor bladeat room temperature to a thickness of about 25 microns. The electrolyteis then irradiated with a 3 Mrad dose of radiation from an electron beamsource. The impedance of the electrolyte layer is about 0.8 ohm/cm².

A 4 cm×12.5 cm×125 micron thick lithium strip (lithium metal/batterygrade) available from Lithco of Bessemer City, NC is applied to one endof the electrolyte layer and the lithium strip is adhered to the layerby applying pressure from a 2 inch diameter, 10 inch long roller at 5-10kg/cm² across the entire lithium surface. The opposite end of theelectrolyte/cathode/current collector assembly is folded over the anodelayer to form a bifaced cell. Copper tabs were spot welded to thecurrent collector and pressure rolled onto the lithium foil to formelectrodes for connection to a device.

The physical and mechanical properties of the produced battery were asfollows:

    ______________________________________                                        property            value                                                     ______________________________________                                        surface area        100 cm.sup.2                                              volume              2 cm.sup.3                                                capacity            250 mAh                                                   average voltage     2.4 V                                                     discharge time (50 mA drain)                                                                      5 hours                                                   discharge time (250 mA drain)                                                                     0.5 hours                                                 discharge time (10 A pulses)                                                                      1.5 minutes                                               energy density      300 wh/l                                                  overall impedance   150 ohm/cm.sup.2                                          ______________________________________                                    

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention defined in the appended claims.

What is claimed is:
 1. A process for making a solid state alkali metalcell capable of generating a discharge current per unit area greaterthan or equal to 50 milliamperes per square centimeter at roomtemperature comprising the steps of:coating a cathode composition layercomprising an active cathode material, an electrically conductive fillermaterial, an alkali metal salt, an inert liquid solvent for said saltand a polymerizable material onto a current collector layer; coating anionically conductive, polymerizable electrolyte composition layer ontosaid cathode composition layer; partially or totally curing said cathodecomposition and said electrolyte composition; and applying an alkalimetal anode composition layer onto said electrolyte composition layer toform a cell assembly.
 2. The cell according to claim 1 wherein saidprocess includes the additional step of applying pressure to said cellassembly to further adhere said layers.
 3. The process according toclaim 2 wherein said process comprises the additional step of rollingthe surface of said cathode composition which is not in contact withsaid current collector layer, said rolling step occurring just prior tosaid electrolyte layer coating step.
 4. The process according to claim 3wherein said curing step comprises partial irradiation or partialthermally curing of said cathode composition layer and said electrolytecomposition layer and wherein said process comprises the additional stepof complete irradiation or complete thermally curing of said cathodecomposition layer and said electrolyte composition layer directly afterapplying said alkali metal anode composition layer.
 5. The processaccording to claim 4 wherein said process comprises the additional stepof rolling the surface of said alkali metal anode composition layerwhich is to contact said electrolyte layer, said rolling step occurringprior to coating said alkali metal anode layer.
 6. The process of claim5 wherein said process is carried out in an atmosphere containing lessthan 4 ppm water.